In recent years, a self-luminous type display device of which pixel is constituted by a light emitting diode (LED) or the like as a display element has attracted attention. As a display element used in such a self-luminous type display device, an organic light emitting diode (OLED which is also referred to as an organic EL element, an electroluminescence (EL) element, or the like) has attracted attention and became to be used in an organic EL display and the like. Since the display element such as an OLED is self-luminous type, the visibility of a pixel is higher as compared to a liquid crystal display, a backlight is not required and the response is fast. In addition, the luminance of the display element is controlled by a current value flowing through it.
As a driving method of controlling the light emission gradation in such a display device, there are a digital method and an analog method. In a digital method, the display element is turned on/off by digital control, and a light emitting time is controlled to express gradation; this method is advantageous in that the uniformity of luminance of each pixel is high, while the frequency is required to be increased and power consumption becomes large. On the other hand, an analog method includes a method of controlling the light emission intensity of the display element by analog control and a method of controlling a light emitting time of the display element by analog control. The method of controlling the light emission intensity by analog control is easily affected by variation of characteristics of a thin film transistor (hereinafter also referred to as a TFT) for each pixel and light emission for each pixel is also varied. Meanwhile, the analog time gradation method controls a light emitting time by analog control, so that the uniformity of luminance of each pixel is high; a display device with the analog time gradation method has been described in Non-Patent Document 1 (see Non-Patent Document 1).
Specifically, in a pixel of a display device described in Non-Patent Document 1, an inverter is constituted by a display element and a transistor for driving the display element. A gate terminal of the driving transistor corresponds to an input terminal of the inverter and an anode of the display element corresponds to an output terminal of the inverter. In the case where an image signal voltage is written into the pixel, the inverter is set to be intermediate between on and off. Then, a triangular wave voltage is inputted into the pixel in a light emitting period to control an output of the inverter. That is, by controlling an output of the inverter which is a potential supplied to the anode of the display element, whether the display element emits light or not is controlled.
[Non-Patent Document 1] SID 04 DIGEST P1394-1397